Page 194 - Phase-Locked Loops Design, Simulation, and Applications

Source : Phase-Locked Loops: Design, Simulation, and Applications, Sixth
Edition Ronald E. Best 119

Mixed-Signal PLL Applications Part 1: Integer-

N Frequency Synthesizers

Synthesizers in Wireless and RF Applications

PLL frequency synthesizers play an ever-increasing role in the field of communications.
Originally, the frequency synthesizer has been a system creating a set of frequencies that
were an integer multiple of a mostly fixed reference frequency. Such synthesizers
(referred to as integer-N frequency synthesizers) are found in every FM radio receiver,
every TV receiver, and the like. Later the fractional-N synthesizer was developed. In
contrast to the integer-N frequency synthesizer, this novel device is able to create
frequencies that are N . f times a reference frequency, where N is the integer part and f is
the fractional part an arbitrary number N. f. Whereas fractional-N synthesizers have been
considered rather “exotic” in the past, they suddenly have gained increased interest,
mainly in mobile communications and in spread-spectrum applications. Fractional-N
frequency synthesizers will be discussed in greater detail in Chap. 7.
Conventional communications used one single carrier whose frequency was fixed.
Radio and TV transmitters are examples for this category. In military communications, it
showed up that such constant carrier frequency links could easily be corrupted by
20
“jammers.” This lead to the development of “frequency hopping.” In frequency-
hopping applications, the single carrier is replaced by a large set of carrier frequencies.
This set consists of a number N of carrier frequencies that are switched in a pseudo-
random manner. This means the transmitter repeatedly jumps through these N carrier
frequencies. The receiver, which must know the carrier frequency sequence of course, has
to track the carrier frequency at any time. Each individual carrier frequency is called a
“chip” in the frequency-hopping vocabulary. The duration of such a “chip” is
usually on the order of several hundred microseconds. This implies that the receiver must
be able to switch the carrier frequency quickly—say, within about 100 μs. It will be
demonstrated in Chap. 7 that fractional-N synthesizers

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